Examine the influence of vulcanicity on drainage in East Africa

 Approach

  • Define vulcanicity

  • Explain the origin/ causes

  • Identify forms of drainage

  • Identify land forms resulting from vulcanicity associated with drainage with specific examples from East Africa.

Answer Guide

Volcanicity is the process through which gases and molten rocks are extruded onto the earth’s surface through a central vent or fissures.

the influence of vulcanicity on drainage in East Africa


Volcanicity is brought about by radio activity and geo-chemical reactions in the interior generating intense/ great heat and pressure; melting mantle rock into molten rock/ magma.

Some times molten magma forms as a result of Friction between moving rocks during plate movement and faulting.

High pressure pushes out the magma into the crust; rising towards the surface through vent and lines of weakness such as faults created by tectonic forces/ movements such as faulting.

The shape and the size of the volcanic landforms are determined by the chemical composition/viscosity of magma and the nature of volcanic eruption.

That is acidic (high silica content), basic magma(less silica) and intermediate

The effects of extrusive volcanicity on drainage features depend on the nature of eruption and the magma ejected. That is;

Violent gaseous explosion removes overlying crystalline rocks to form wide circular depressions that are filled with water to form explosion crater lakes for example Lake Katwe, Nyamunuka, Kasenyi, Kikorongo, e.t.c in south western Uganda.

Violent eruption may blow off the top / summit of a volcano or the summit of a volcanic mountain may collapse into the chasm to form a smaller circular depression filled with water to form mountain crater lake for example Lake Katungi in Bushenyi, Gisozi in Kisoro, crater lakes on Mount Elgon, Kenya, Kilimanjaro and Muhavura, Menengai crater in Kenya’s rift valley and Lake Simbi in south- Nyanza district (Kenya) and Lake Paradise on the summit of mount Marsabit.

Crater lakes are usually very small.

Some times violent eruption or subsidence of the summit forms a large, wide and circular depression on the volcanic mountain filled with water to form caldera lakes for example Lake Ngozi, Embagai and Ngorongoro in Tanzania.

Lava dammed lakes are formed as a result of deposition of basic lava across the existing river valleys causing damming and back ponding of the river for example lake Bunyonyi, Mutanda, Kyahifi, Kayumbo and Murehe in Kigezi south western Uganda, Lake Saka in Fort Portal.
Violent eruption of acidic lava builds Volcanic cones or mountains which form divides/water shed for numerous rivers forming radial drainage whose subsequent streams develop dendritic pattern for example Manafwa, Sironko, Malaba and Nzoia on mount Elgon and those on mountain Kenya, Kilimanjaro and Muhavura.

Volcanic materials that are porous in nature do limit surface drainage for example the basalts in Kisoro area and Bunyaruguru lack flowing rivers/surface drainage.

Intrusive volcanicity influences drainage especially where formerly existing dykes and seals are exposed by continuous erosion for example Waterfalls and rapids develop where rivers cross resistant intrusive land forms such as dykes and sills. For example Sipi falls in Kapchorwa and Kisiizi falls in Rukungiri.

The presence of waterfalls and rapids lead to formation of other related features for example gorges, plunge pools e.t.c formed by falling water on soft rocks. Murchison gorge Hot springs- mass of hot underground water flowing to the surface of the earth through fissures

Geysers- mass of underground steam and hot water gushing out to the surface through fissures. Hot springs and geysers partly owe their origin to vulcanicity.

Due to the existence of hot rocks or molten rock beneath the surface of the earth, when it rains water sinks into the ground and comes in to contact with super heated rocks and is therefore heated. Pressure builds up and steam and hot water expand in the fissures, rising to the surface. Hot springs are evident in Bundyibugyo district like Sempaya, Kitagata in Bushenyi and Kisiizi in Rukungiri.

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